A=(({{10, -2, 1}, {0.8, -24, 3}, {0, 4, 58}}))

b={2,21,-1.2}

LinearSolve[A,b]

A[[1,1]]

A[[3,2]]

A[[3,2]]

A[[3]]

(* инициализираме празни матрица C и вектор d: *)
c=(({{\[Placeholder], \[Placeholder], \[Placeholder]}, {\[Placeholder], \[Placeholder], \[Placeholder]}, {\[Placeholder], \[Placeholder], \[Placeholder]}}));d={0,0,0};

n=Length[A];
For[i=1,i≤ n,i++,
c[[i]]=((-A[[i]])/(A[[i,i]]));
c[[i,i]]=0;
d[[i]]=b[[i]]/A[[i,i]]
]
c//MatrixForm
d

b

Norm[c]